Carbohydrate Deficient Glycoprotein Syndromes (CDGS) are genetic defects in N-glycosylation that cause severe neurological lesions and developmental delay. Two primary defects in CDGS are known, but many others are unknown. We propose to identify some of them by well- established and previously successful approaches. In addition, we discovered a non-neurological form of CDGS that presents with severe life- threatening protein-losing enteropathy, hypoglycemia, and intestinal bleeding. The primary defect is a 95% deficiency in phosphomannose isomerase (PMI, Fru-6< >Man-6-P). It is important to analyze additional patients with this type of CDGS since dietary mannose supplements effectively reverse all the clinical and biochemical anomalies. Ongoing Phase I studies of mannose therapy cover both neurological and gastrointestinal (PMI-deficient) patients. A simple isoelectric focusing test of serum transferrin is diagnostic for all types. To identify new defects and analyze additional PMI-deficient patients we plan to: 1. Analyze the structure of sugar chains on carbohydrate-deficient serum transferrin as an indicator of possible primary glycosylation defects. 2. Metabolically label patient fibroblasts with sugar precursor to assess protein glycosylation and all biosynthetic intermediates. 3.Based on results from im 2, directly assay biosynthetic enzymes likely to be deficient. If the defect occurs in known or highly conserved biosynthetic enzymes, identify the specific genetic lesions. 4. Identify mutations in PMI activity-deficient patients and relate these to their clinical conditions. These tried-and-true approaches have been highly successful for identify the defects of glycosylation mutants in man, mammalian cells and yeast. In fact, no second option is currently available. This is the first broad-based proposal to study CDGS defects, and by interfacing it with the mannose trials, it merges basic science and clinical medicine. As we learn more about these disorders and therapies, we expect to discover additional types of CDGS. Analysis of glycosylation-deficient patients will broaden our understanding ob both glycobiology and its immediate application to human glycosylation diseases.